1. Field of the Invention
This invention pertains to methods and apparatus for introducing an oxidizing oxygen-fuel combustion in air-fuel fired furnaces to reduce NOx emissions and improve thermal efficiency without any substantial detrimental effect on furnace life or product quality.
2. Related Art
The most common method to reduce NOx emissions is to use 100% oxygen-fuel combustion where use of oxygen instead of air eliminates nitrogen and thus significantly lower NOx emissions are achieved. This method has been successfully used on several types of glass furnaces. However, the use of 100% oxygen-fuel firing on large glass furnaces (450 to 1000 ton/day melted glass production capacity), such as float glass furnaces, has not been achieved so far due to overall economics with the oxygen use and uncertainty over glass quality and furnace life.
There are several other NOx control methods available in the market such as 3-R process (European Patent No. 0 599 547 A1), gas reburn process (U.S. Pat. No. 5,139,755) and oxygen-enriched air staging (U.S. Pat. No. 5,203,859).
The 3-R process and the gas reburn process use additional or reburn fuel injection (5% to 15% of total fuel use) in the exhaust stream to create gas reburn reactions and reduce NOx emissions. This is a post combustion method. This method requires injecting reburn fuel (natural gas) in the exhaust stream, which may be difficult for certain furnaces which cannot use reducing conditions in the regenerator due to refractories containing various oxides. Further the reburn fuel is an energy penalty and no thermal efficiency benefit is derived by injection of 5% to 15% reburn fuel in the exhaust stream outside the melt area. Here the additional fuel does not release any heat for the productivity increase and it is simply used as emissions cleaning medium. There are also concerns of higher CO emissions from the furnace.
In oxygen-enriched air staging, the secondary oxidant (oxygen or oxygen-enriched air) is introduced proximate to the exhaust of the industrial furnace to reduce NOx emissions. In these applications the furnace is operated using lower stoichiometry on the firing side to reduce thermal NOx formation. The secondary oxidant is injected into the exhaust stream (using exhaust port) to burnout CO and other hydrocarbons. This concept is illustrated in FIG. 1. In FIG. 1 a typical side fired regenerative furnace 1 having regenerators (checkers) A and B is illustrated schematically with both firing 2 and exhaust 4 ports. The firing is from left-to-right and secondary oxidant injection 6 is from right-to-left.
In U.S. Pat. No. 5,203,859 (as illustrated in FIG. 1), the preferred embodiment includes withdrawal of preheated secondary combustion air 7 from firing side regenerator using an oxygen aspirator 8. The oxygen is used as a prime mover for withdrawing secondary combustion air 7. The secondary oxidant 6 is then injected proximate to the exhaust 4. The disadvantages of above scheme include:
Space constraints due to large secondary air piping 7 carrying 2400.degree. F. [1315.degree. C.] air. PA1 Complex flow reversal cycle to switch secondary preheated air from left-to-right side depending on the reversal cycle. PA1 Difficulties in the burnout of CO and hydrocarbons in the melter space due to premature combustion in the exhaust port 4 leading to overheating of exhaust port 4. PA1 Design limitations of aspirator 8 in providing correct secondary oxidant mixture PA1 High capital cost system. PA1 a) combusting a first fuel in at least one air-fuel burner, heat from the air-fuel burner being substantially transmitted to the load; PA1 b) combusting a second fuel in at least one oxy-fuel burner, heat from the oxy-fuel burner being substantially transmitted to the load; wherein the air-fuel burner is operated in fuel-rich mode, and the oxy-fuel burner is operated in fuel-lean mode. PA1 a) operating one or more air-fuel burners at constant fuel input; and PA1 b) operating one or more oxy-fuel burners to increase or decrease temperature of the load without substantially changing production of NOx from the furnace. PA1 a) a central conduit adapted to deliver an oxidant; PA1 b) an annular region external of the central conduit, the annular region adapted to deliver a fuel; PA1 c) the central conduit having a nozzle attached at a central conduit end, wherein either the nozzle or the central conduit are adapted to be adjusted axially.
Other known embodiments of the above include secondary oxidant as oxygen-enriched ambient air, which would create difficult mixing conditions due to smaller relative volume of the cold or ambient oxidant stream compared to primary exhaust stream. Here the exhaust gas volume is approximately 60 times greater than the secondary oxidant leading to inefficient mixing and poor thermal efficiency due to quenching of the melter combustion space by an ambient mixture. This can further result in poor product quality.
Additional known embodiments include use of oxygen as a secondary oxidant, which also creates difficult mixing conditions due to small gas volume (300 times smaller than the primary exhaust stream). This creates non-homogeneous burnout and creation of hot spots in the melter combustion space and exhaust port.
It would be a great benefit to glass and other manufacturers if NOx production could be decreased, while transferring heat to the load and avoiding some of the problems mentioned above.